Electrocoating, or electrodeposition coating, is a means of applying a coating to a conductive article or workpiece. In the electrocoating process, the conductive article that is to be coated is used as one electrode in an electrochemical cell. The article is submerged in an aqueous dispersion of the coating composition, which contains a charged, preferably a cationic, resin. The resin is deposited onto the article by applying an electrical potential between the article and a second electrode. The coating deposits onto the article until it forms an insulating layer on the article that essentially prevents more current from being passed.
The electrocoating process is particularly suited to applying a continuous and uniform protective primer layer to an article or workpiece that has complex shape or construction. When the surfaces of the article closest to the other electrode have been coated and insulated, the current deposits the coating onto recessed areas and other less accessible areas until an insulating coating layer is formed on all conductive surfaces of the article or workpiece, regardless of how irregularly shaped the article is.
Electrocoat processes, particularly for coating automotive bodies and parts, usually employ a thermosetting coating composition comprising a cationic principal resin and a polyfunctional oligomeric or monomeric crosslinking agent that is capable of reacting with the principal resin under curing conditions. The crosslinking agent is associated with the principal resin in the dispersion and is deposited along with the principal resin onto the article or workpiece. After deposition, the deposited coating may be cured to a crosslinked, durable coating layer.
Although other crosslinking agents, such as aminoplast resins, have been used, polyisocyanate crosslinking agents are predominantly used and preferred in automotive electrocoat applications, in which the workpieces are, for example, vehicle bodies, wheel rims, and other metal parts. The polyisocyanate crosslinking agents react with hydroxyl groups on the principal resin to form urethane linkages, or with primary or secondary amine groups on the principal resin to form urea linkages. Urethane and urea linkages are preferred in automotive and other applications because of the durability and hydrolytic stability of such linkages.
There are, however, a number of disadvantages in using polyisocyanate crosslinking agents. One such disadvantage is that, in order to prevent the reaction of the isocyanate groups of the crosslinking agent with water in the dispersion, or the premature reaction with the principal resin, the isocyanate groups must be reversibly blocked before the crosslinking agent is added to the coating composition. Besides the time and expense of the extra blocking step, high temperatures (usually 150.degree. C. or more) are required to reverse the blocking reaction and regenerate the isocyanate groups before they can react to crosslink the principal resin. Moreover, the volatile blocking agents released during this reversal can cause deleterious effects on coating properties, as well as increasing undesirable air emissions from the process. Another drawback is that the toxicity of monomeric isocyanates, particularly aromatic isocyanates, requires special handling procedures during manufacture of the blocked polyisocyanate crosslinking agents. Additionally, aromatic isocyanates have been associated with film yellowing.
It would be desirable to have a method of forming the durable urethane and urea linkages during the cure of the electrocoat film without the attendant problems of blocked polyisocyanate curing agents. We have now discovered an improved method for producing durable, crosslinked electrocoat films.